The invention is based on a priority application EP 01440101.2, which is hereby incorporated by reference.
The invention relates to a method to ensure the quality of preferred (communications) services, especially voice services, in a local data network with different types of services, wherein data congestion or data collision on a shared transmission medium is treated by a collision detection method and wherein different re-transmission rules are applied corresponding to the type of service, as well as to a local network, a station, a local network controller an a program module therefor.
In Internet Telephony voice is transported over a network using data packets instead of circuit switched connections over voice only networks. Internet Telephony or IP Telephony refers to the transfer of voice over the Internet Protocol (VOIP) of the so-called TCP/IP (Transfer Control Protocol/Internet protocol) protocol suite.
In an advanced corporate enterprise, IP telephone terminals connect via a Local Area Network (LAN), often based on the so-called Ethernet, along with Personal Computers. These IP telephones interact with a VOIP-enabled private exchange (PBX) for call set-up and administration and access to the external (public switched) telephone network (PSTN) and/or an external packet network, especially the internet.
The transmission medium or bus of an Ethernet system is a shared medium, where transmission is not controlled by a central control unit, e.g. a bus master, but all terminals or stations connected follow certain rules, when transmitting data on the transmission medium. These rules are described in the document “IEEE Std 802.3, Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications”. All stations are always listening the transmission medium for data traffic and they wait for sending data until said data traffic has ceased. If, at one time, two ore more stations start sending data on the transmission medium, data collision occurs. Said stations, detecting the collision, stop sending and wait each for a specific random time until starting re-transmission. The detection of a collision may be carried out by supervision of the voltage of received signals. See following example: If the data signal sent on the bus shows two voltage levels, e.g. 0 Volt for a bit representing the value “0” and 5 Volt for a bit representing the value “1”, a collision is detected in a receiving station as soon as said station sees a signal voltage, that exceeds the defined maximum voltage of the data signals, i.e. 5 Volt, caused by superposition of two or more data signals.
For a reliable and fast detection of collisions, any station detecting a collision may explicitly send out a specific information, a so-called jam signal. As soon as this jam signal is received by the sending stations, said stations stop sending. Without limiting the invention, it is assumed in the following, that an explicit jam signal is sent by the stations, when a collision is detected.
The more stations are connected to the transmission medium and the more these stations are actively sending data, the probability for data collision increases; it might happen, that one station has to start re-transmission several times until data transmission can be successfully completed. For avoiding dead locks evoked by peak situations of bursty data traffic, the number of re-transmission trials can be limited. When exceeding this limit, the affected data packets are dropped.
In the following, only the Ethernet is considered for the two lowest layers of a local network, the data link level and the physical level of the so-called OSI reference model. Nevertheless, the invention can be carried out in any network with a shared (transmission) medium, where transmission on this medium is controlled by the stations themselves.
In classical local area networks based on Ethernet, generally all data is treated uniquely. The previously described mechanism of collision does not distinguish between different types of data. But more and more, additionally to “normal data”, e.g. data to be visualised on a display, real time data traffic, especially voice (data) traffic for voice over IP applications, is to be treated by local area networks.
Real time communication applications exhibit strict latency requirements. A lot of real time communication application require, that time delay between sending and receiving data, must not exceed a defined time value. Voice over IP applications, e.g., require a maximum delay of about 0, 2 seconds. Data exceeding a defined delay time is dropped, i.e. is not used for communication. To guarantee high quality real time communication services, the drop rate, i.e. the ratio between the number of dropped data packets and the number of transmitted data packets, has to be kept low.
For economic reasons, it is required, to both handle normal data traffic and real time data traffic in a unique local network. But today no method of different treatment of both types of data in local networks with a shared medium exists.
U.S. Pat. No. 6,172,984 discloses a system and method for reducing the latency for time sensitive data over the shared medium of a local network. A station ready to send time sensitive data sends a collision signal on the bus and immediately proceeds with the high priority transmission. This method, however, does not solve the problem of collisions between high priority data itself.